A monolithic honeycomb substrate will typically have an inlet end and an outlet end, with multiple mutually adjoining cells extending along the length of the body from the inlet end to the outlet end. These honeycomb substrates typically have from about 100 to 600 cells-per-square-inch (cpsi), but may have densities range from 10 cpsi to 1200 cpsi. Cells having round, square, triangular, or hexagonal cell shapes are known.
The open frontal area may comprise 50% to 85% of the surface area, and the cell wall thickness may be from 0.5 to 10 mils, where 1 mil is 0.001 inches. The cells also may be separated from one another by walls with a thickness in the range of about 0.5 mil to about 60 mils (0.012 mm to 1.5 mm). In some cases the open frontal area may be as much as 91% for a 600 cpsi substrate with 2 mil cell wall thickness.
The cell walls of the substrate may be porous or non-porous, smooth or rough. For porous walls, an average wall pore diameter may be from about 0.1 to about 100 microns, and wall porosity may typically range between about 10-85%.
Under various operating conditions, carbon deposits (e.g., soot, coke) may accumulate on the leading edges of the cell walls. Over time this build-up of soot and coke can reduce the size of the cell openings and the overall open frontal area. This reduction in opening size can lead in turn to increased back pressure and lower gas flow through the catalytic substrates.